ADSORPTION OF WATER-VAPOR BY HOMOIONIC MONTMORILLONITES - HEATS OF ADSORPTION AND DESORPTION

Adsorption isotherms for water vapor, basal spacing, and immersion hea ts and water desorption heats of Li+, Na+, Mg2+, Ca2+, Cu2+, and Fe3montmorillonite are measured at various relative humidities (r.h.). Th e amount of water adsorbed as a function of r.h. is found to increase gradually, whereas basal spacing increases and the adsorption heat dec reases in steps. The water desorption heat also decreases in steps. Th e entropy of adsorbed water appears to be negative with respect to the entropy of liquid water. A theoretical model is proposed to describe the hydration process Of Li+, Na+, Mg2+, Ca2+, Cu2+, and Fe3+ montmori llonites. The experimental adsorption heats are found to have a direct relationship with the sum of the hydration energy of the cations plus expansion energy. (C) 1997 Academic Press